1. Field of the Invention
The present invention relates to an optical transmitter used in wavelength division multiplexing optical communications, and particularly, to an optical transmitter for coupling and wavelength-multiplexing optical signals with a different wavelength by means of reflective and transmissive optical components.
2. Description of the Related Art
In the field of optical communications, the prevailing optical transmission method that expands information capacity is that by wavelength division multiplexing (WDM), in which a plurality of signals are carried by light of separate wavelengths, each of which is multiplexed and transmitted through one optical fiber.
FIG.2 is a schematic view illustrating an optical transmitter using a waveguide optical coupler.
As shown in FIG.2, an optical transmitter 70 comprises a light source portion 71 and an optical coupling portion 72. The light source portion 71 comprises a circuit board 73, and 4 light sources 74-77 arranged with a specified spacing. On the circuit board 73, there are packaged an IC, and electronic components such as a resistor, capacitor, etc., not shown, for controlling optical power of the light sources 74-77. The optical coupling portion 72 comprises a planar waveguide optical coupler, in which 2 cores extending from 4 inlet ends 81-84 are coupled together in Y-coupling portions 85 and 86, and in which the coupled waveguides are coupled together in a Y-coupling portion 87, and communicate with an outlet end 88.
Optical signals with a different wavelength from the light sources 74-77 are respectively passed into the 4 inlet ends 81-84, and combined together in the Y-coupling portions 85-87, and a wavelength multiplexing optical signal with the 4 different wavelengths combined together is emitted from the outlet end 88.
Because the optical transmitter 70 comprises the light source portion 71 and the planar waveguide optical coupling portion 72, it has fewer components and simple structure (See JP-A-2003-14994, for example).
However, because the optical transmitter 70 has the planar waveguide optical coupler formed as the optical coupling portion 72, its polarization-dependent loss of optical signals propagating through the waveguide becomes large. Also, because the optical transmitter 70 has the planar waveguide optical coupling portion 72, it has a low manufacturing yield. In the case of an optical transmitter for a 4-wavelength multiplexing optical signal, since there are packaged 4 LDs as the light sources 71-74, the spacing between the waveguides of the optical coupling portion 72 has to be large, which results in a large-size optical coupling portion 72 and therefore a high cost.
Accordingly, there is an optical transmitter 60 for coupling optical signals using optical components, as shown in FIG.1.
The optical transmitter 60 comprises 4 light sources 61-64 formed in an array for emitting respective parallel optical signals L61-L64 with a different wavelength, and respective optical components 65-68 arranged in a line in an optical path of the optical signal emitted from each light source.
The optical component 65 arranged in the optical path of the optical signal L61 reflects the optical signal L61 emitted from the light source 61. The optical component 66 arranged in the optical path of the optical signal L62 not only reflects the optical signal L62, but also transmits an optical signal 161 reflected by the optical component 65 to couple the optical signals L62 and 161. The optical component 67 arranged in the optical path of the optical signal L63 not only reflects the optical signal L63, but also transmits the optical signal 162 coupled at the optical component 66 to couple the optical signals L63 and 162. The optical component 68 arranged in the optical path of the optical signal L64 not only reflects the optical signal L64, but also transmits the optical signal 163 coupled at the optical component 67 to couple the optical signals L64 and 163. An optical signal L coupled at the optical component 68 is emitted as a 4-wavelength multiplexing optical signal.
Here, to equalize optical signal power of each optical signal L61-L64 of the optical signal L coupled at the optical component 68, the reflectivity of the optical component 65 is 100%, the reflectivity of the optical component 66 is 50%, the reflectivity of the optical component 67 is 33%, and the reflectivity of the optical component 68 is 25%.
For the optical signal L61, 100% of optical signal power is reflected at the optical component 65, 50% thereof is transmitted at the optical component 66, 67% thereof is transmitted at the optical component 67, and 75% thereof is transmitted at the optical component 68, to be coupled to the other optical signals L62, L63 and L64. Therefore, the optical signal power of the optical signal L61 transmitted through the optical component 68 is:1×0.5×0.67×0.75=0.25,which is 25% of optical signal power of the light source 61.
For the optical signal L62, 50% of optical signal power is reflected at the optical component 66, 67% thereof is transmitted at the optical component 67, and 75% thereof is transmitted at the optical component 68, to be coupled to the other optical signals L61, L63 and L64. Therefore, the optical signal power of the optical signal L62 transmitted through the optical component 68 is:0.5×0.67×0.75=0.25,which is 25% of optical signal power of the light source 62.
For the optical signal L63, 33% of optical signal power is reflected at the optical component 67, and 75% thereof is transmitted at the optical component 68, to be coupled to the other optical signals L61, L62 and L64. Therefore, the optical signal power of the optical signal L63 transmitted through the optical component 68 is:0.33×0.75=0.25,which is 25% of optical signal power of the light source 63.
For the optical signal L64, 25% of optical signal power is reflected at the optical component 68, to be coupled to the other optical signals L61, L62 and L63.
In this manner, the 4-wavelength multiplexing optical signal L consists of a signal in which 25% of each optical signal L61-L64 is coupled. And the optical transmitter 60 wavelength-multiplexes optical signal power of each wavelength equally by making the reflectance of the optical components 65-68 different.
However, because in the optical transmitter 60 shown in FIG.1, equalizing wavelength-multiplexed optical signal power requires making the respective reflectance of each optical component 65-68 different, there is the problem that there are many kinds of optical components used, which therefore results in a high manufacturing cost.
The other related art is for example JP-A-2003-195119.